I wrote an example of Bob Nystrom’s "Baby’s First Garbage Collector," which I’ve been wanting to implement for a while in order to understand it better. To make the problem harder (as I always do), I decided to write it in C++, and to make it even more fun, I’ve implemented it using the new Variant container from C++17.

I’ve never written a garbage collector before. Now I know what it is and how it works.

The collector Nystrom wrote is a simple bi-color mark-and-sweep collector for a singly threaded process with distinct pauses based upon a straightforward memory usage heuristic. That heuristic is simply, "Is the amount of memory currently in use twice as much as the last time we garbage collected?" If the answer is yes, the collector runs and sweeps up the stack.

Nystrom’s code is highly readable, and I hope mine is as well. Because I used Variant, my Object class has an internal Pair class, and then the Variant is just <int, Pair>, where "Pair" is a pair of pointers to other objects. The entirety of the VM is basically a stack of singly-linked lists which either represents integers or collections of integers in a Lisp-like structure.

The allocator creates two kinds of objects, then: pairs, and lists. A pair is created by pushing two other objects onto the stack, then calling push(), which pops them off the stack and replaces them with a Pair object. The VM class has two methods, both named push(), one of which pushes an integer, the other a pair. Since a pair is built from objects on the stack, the Pair version takes no arguments, and since C++14 and beyond have move semantics that Variant honors, Variant<Pair> only constructs a single pair. Pretty nice. I was also able to use both lambda-style and constructor-style visitors in my Variant, which was a fun little bonus.

In the end, this becomes a pair of linked lists with different roots; one pair can orphan the lists, the other can’t. We traverse up the spine of the stack, following each scope’s list and marking the objects as found. We then traverse the other list and, for every object not marked, we get a pointer to it, collapse the list around it, and delete it. Very simple and elegant.

I hope, eventually, to move onto tri-color, multi-threaded garbage collectors someday. According to Nystrom, this garbage collector is the actual algorithm used in early versions of Lua, so it’s not a toy.

I have included the header files for the Mapbox version of Variant since the C++17 committee’s standards haven’t quite reached the general public and the Variant implementation is still a subject of some debate. This implementation looks straightforward enough and is a header-only release. It works with both GCC 4.8.5 and Clang 3.8, and that’s good enough for me.

The Mapbox variant is BSD licensed, and a copy of the license is included in the Include directory.

BUILDING:

From the base directory of the project:

mkdir build
cd build
cmake ..
make

And you should be able to run the basic tests. It’s just one file. Unfortunately, I was simpleminded with the include paths, so it can’t be built anywhere but from the base directory without fiddling with the CMakeLists.txt file.